The invention relates generally to devices and methods for trapping and sequestering non-aqueous-phase liquids (NAPLs) from subaqueous sediment. NAPLs are hydrophobic liquids having low solubility and a low surface tension. Consequently they have a low water solubility and form a separate phase liquid when placed in water. NAPLs can be lighter-than-water (LNAPL) or denser-than-water (DNAPL). NAPLs are often found at or near industrial sites or former industrial sites. For example, historic wastewater outfalls from manufactured gas plants (MGP) sites often contain NAPL-contaminated sediment from the discharge of coal tar. Likewise, current or historical discharges from other industrial sites, such as refineries, wood-treating facilities, asphalt plants, coking plants, steel mills, and others may contain NAPL (either or both LNAPLs and DNAPLs).
Buoyant gases (e.g. carbon dioxide, methane) produced upon degradation of organic matter present in underwater sediment can migrate as buoyant bubbles via ebullition to the surface of a body of water. When the underwater sediment contains a NAPL, the NAPL can be entrained with the bubbles and carried upward through the sediment and through the overlying water column to the water surface. After the bubble breaks at the water surface, the entrained NAPL can form at the water surface a sheen that can pose a health hazard to human and ecological receptors (e.g., coal tar and gasoline contain known human carcinogens). LNAPL, being lighter than water, can itself rise through the water without being entrained with the buoyant gas bubbles toward the surface where it too can form a surface sheen.
NAPL migration has been addressed by dredging, in-filling, in-situ stabilization, and capping. Published US Patent Application 20090110486, incorporated herein by reference in its entirety as if set forth herein, describes NAPL trapping cap devices and methods for use. The art is still in need of additional devices and methods for controlling or preventing NAPL migration from NAPL-contaminated sediments upward toward the surface, particularly where it is difficult to direct LNAPL or DNAPL to an accumulation zone or to vent gases present in the contaminated sediment at the accumulation zone. Such conditions can exist when the contaminated subsurface sediment is located far from shore or over uneven underwater surfaces.